1. Field of the invention
The present invention relates to a semiconductor device and a method for manufacturing the semiconductor device. Specifically, the present invention relates to a semiconductor device that can be manufactured in a reduced cost by decreasing the number of masks, and a method for the semiconductor device.
2. Related Art
A transistor is provided over a substrate that constitutes a liquid crystal display device. As the transistor, a pixel transistor that is required to display an image and a drive circuit transistor that is required to drive the pixel transistor are used. For the pixel transistor, a LDD (Lightly Doped Drain) transistor is used to reduce an OFF current. For the drive circuit transistor, a gate overlap LDD (GOLD) transistor that requires high load current is used to obtain high reliability and a high ON current.
In order to manufacture a large panel, it is necessary to use a low resistance wiring for a source wiring and a gate wiring of a pixel portion. Therefore, a low resistance wiring is used as a gate electrode, or a low resistance wiring is separately provided.
Reference 1: Unexamined Patent Publication No. 2001-290171 (pp. 6 to 19, FIGS. 1 to 5)
For example, in order to manufacture a transparent liquid crystal panel by only an N-channel transistor for the simplification of processes, at least six masks are required when a GOLD transistor and a LDD transistor are simultaneously manufactured as described above.
In the case that aluminum (Al) that has low resistance is used as a gate electrode material, heat treatment at high temperature can not be carried out after forming an impurity region of a transistor since the Al has low heat-resisting properties. Further, in the case that a gate electrode in a GOLD structure that overlaps a LDD region is formed, it becomes difficult to activate the LDD region overlapped with the gate electrode by laser irradiation. However, the reliability of a transistor may be decreased without the activation.
It is also possible that a GOLD structure is manufactured by a low resistance wiring after an impurity region is sufficiently heat activated by forming a gate electrode by a material having heat-resisting properties. However, six masks are also required in this instance, and it is difficult to reduce the number of masks. In order to reduce the manufacturing cost in future, the number of masks is required to be further reduced.